Re: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc for the ARM SMC/HVC mailbox

[Andre Przywara <andre.przywara@arm.com>]

On Tue, 10 Sep 2019 21:36:35 -0500
Jassi Brar <jassisinghbrar@gmail.com> wrote:

Hi,

> On Mon, Sep 9, 2019 at 8:32 AM Andre Przywara <andre.przywara@arm.com> wrote:
> >
> > On Fri, 30 Aug 2019 03:12:29 -0500
> > Jassi Brar <jassisinghbrar@gmail.com> wrote:
> >
> > Hi,
> >  
> > > On Fri, Aug 30, 2019 at 3:07 AM Peng Fan <peng.fan@nxp.com> wrote:  
> > > >  
> > > > > Subject: Re: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc for the ARM
> > > > > SMC/HVC mailbox
> > > > >
> > > > > On Fri, Aug 30, 2019 at 2:37 AM Peng Fan <peng.fan@nxp.com> wrote:  
> > > > > >
> > > > > > Hi Jassi,
> > > > > >  
> > > > > > > Subject: Re: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc
> > > > > > > for the ARM SMC/HVC mailbox
> > > > > > >
> > > > > > > On Fri, Aug 30, 2019 at 1:28 AM Peng Fan <peng.fan@nxp.com> wrote:
> > > > > > >  
> > > > > > > > > > +examples:
> > > > > > > > > > +  - |
> > > > > > > > > > +    sram@910000 {
> > > > > > > > > > +      compatible = "mmio-sram";
> > > > > > > > > > +      reg = <0x0 0x93f000 0x0 0x1000>;
> > > > > > > > > > +      #address-cells = <1>;
> > > > > > > > > > +      #size-cells = <1>;
> > > > > > > > > > +      ranges = <0 0x0 0x93f000 0x1000>;
> > > > > > > > > > +
> > > > > > > > > > +      cpu_scp_lpri: scp-shmem@0 {
> > > > > > > > > > +        compatible = "arm,scmi-shmem";
> > > > > > > > > > +        reg = <0x0 0x200>;
> > > > > > > > > > +      };
> > > > > > > > > > +
> > > > > > > > > > +      cpu_scp_hpri: scp-shmem@200 {
> > > > > > > > > > +        compatible = "arm,scmi-shmem";
> > > > > > > > > > +        reg = <0x200 0x200>;
> > > > > > > > > > +      };
> > > > > > > > > > +    };
> > > > > > > > > > +
> > > > > > > > > > +    firmware {
> > > > > > > > > > +      smc_mbox: mailbox {
> > > > > > > > > > +        #mbox-cells = <1>;
> > > > > > > > > > +        compatible = "arm,smc-mbox";
> > > > > > > > > > +        method = "smc";
> > > > > > > > > > +        arm,num-chans = <0x2>;
> > > > > > > > > > +        transports = "mem";
> > > > > > > > > > +        /* Optional */
> > > > > > > > > > +        arm,func-ids = <0xc20000fe>, <0xc20000ff>;
> > > > > > > > > >  
> > > > > > > > > SMC/HVC is synchronously(block) running in "secure mode", i.e,
> > > > > > > > > there can only be one instance running platform wide. Right?  
> > > > > > > >
> > > > > > > > I think there could be channel for TEE, and channel for Linux.
> > > > > > > > For virtualization case, there could be dedicated channel for each VM.
> > > > > > > >  
> > > > > > > I am talking from Linux pov. Functions 0xfe and 0xff above, can't
> > > > > > > both be active at the same time, right?  
> > > > > >
> > > > > > If I get your point correctly,
> > > > > > On UP, both could not be active. On SMP, tx/rx could be both active,
> > > > > > anyway this depends on secure firmware and Linux firmware design.
> > > > > >
> > > > > > Do you have any suggestions about arm,func-ids here?
> > > > > >  
> > > > > I was thinking if this is just an instruction, why can't each channel be
> > > > > represented as a controller, i.e, have exactly one func-id per controller node.
> > > > > Define as many controllers as you need channels ?  
> > > >
> > > > I am ok, this could make driver code simpler. Something as below?
> > > >
> > > >     smc_tx_mbox: tx_mbox {
> > > >       #mbox-cells = <0>;
> > > >       compatible = "arm,smc-mbox";
> > > >       method = "smc";
> > > >       transports = "mem";
> > > >       arm,func-id = <0xc20000fe>;
> > > >     };
> > > >
> > > >     smc_rx_mbox: rx_mbox {
> > > >       #mbox-cells = <0>;
> > > >       compatible = "arm,smc-mbox";
> > > >       method = "smc";
> > > >       transports = "mem";
> > > >       arm,func-id = <0xc20000ff>;
> > > >     };
> > > >
> > > >     firmware {
> > > >       scmi {
> > > >         compatible = "arm,scmi";
> > > >         mboxes = <&smc_tx_mbox>, <&smc_rx_mbox 1>;
> > > >         mbox-names = "tx", "rx";
> > > >         shmem = <&cpu_scp_lpri>, <&cpu_scp_hpri>;
> > > >       };
> > > >     };
> > > >  
> > > Yes, the channel part is good.
> > > But I am not convinced by the need to have SCMI specific "transport" mode.  
> >
> > Why would this be SCMI specific and what is the problem with having this property?
> > By the very nature of the SMC/HVC call you would expect to also pass parameters in registers.
> > However this limits the amount of data you can push, so the option of reverting to a
> > memory based payload sounds very reasonable.
> >  
> Of course, it is very legit to pass data via mem and many platforms do
> that. But as you note in your next post, the 'transport' doesn't seem
> necessary doing what it does in the driver.

Yes, indeed. I didn't realise that until looking more deeply into the driver later.
So I think we are on the same page regarding this: the *controller* driver and its binding does not need to know about the transport, that's something between the mailbox client and the firmware implementation.

Cheers,
Andre.

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